1. Field of the Invention
The present invention relates to a method of preparing a polycarbonate resin, the method including a condensation polymerization for decreasing a mole fraction of arylcarbonate existing in a non-reacted diarylcarbonate, and in a terminated group of a reaction side product having a degree of polymerization of less than 3 produced through transesterification, thus maximizing an increase of the molecular weight of the polycarbonate resin, such as a siloxane-based polycarbonate or a polyester carbonate, produced through solid state polymerization, and substantially reducing the preparation time for the polycarbonate resin.
2. Description of the Related Art
Polycarbonate resins have excellent heat resistance, impact resistance, mechanical strength, and transparency. Due to these advantages, polycarbonate resins are used in various applications, such as compact discs, transparent sheets, packaging materials, mobile bumpers, and ultraviolet (UV)-blocking films, and thus, the demand for polycarbonate resins is increasing.
However, polycarbonates have low solvent resistance, and low impact resistance at low temperature. For example, when polycarbonates are exposed to commonly used solvents, crazing or cracking occurs. Many efforts have been made to overcome these problems, and various modified polycarbonates have been developed. In particular, siloxane-based polycarbonates and polyester carbonates exhibit superior low-temperature impact resistance, molding, and fluidity.
Conventional manufacturing processes for polycarbonate may be categorized into interfacial polymerization processes in which phosgene is used, and melt condensation polymerization processes and solid state polymerization processes in which phosgene is not used.
An interfacial polymerization process, as disclosed in U.S. Pat. No. 5,530,083, includes an operation of reacting an aromatic hydroxy compound, a dihydroxy compound, phosgene, and a catalyst with hydroxyaryl terminated polydioganosiloxane. In this case, high-molecular weight siloxane-based polycarbonate resins can be relatively easily produced by a continuous process. However, the process is accompanied by a risk caused by the use of poisonous gases and a chloride-based organic solvent, which is an environmental pollutant, and thus, the manufacturing costs are high.
A melt condensation polymerization process, as disclosed in U.S. Pat. Nos. 6,252,013 and 6,232,429, includes an operation of performing polymerization to produce polyester carbonate after a starting material is dissolved. The melt condensation polymerization process is relatively stable because a poisonous material is not used. However, in order to produce high-molecular weight polycarbonate for extrusion, a reaction between elements with high viscosity must be handled at a high temperature and in a high vacuum, and these conditions bring about a low quality of polycarbonate.
A solid state polymerization process is a process in which a low-molecular weight polycarbonate prepolymer is crystallized and the crystallized prepolymer is polymerized at a temperature lower than a melting point of the crystallized prepolymer. In this case, a poisonous material is not used, and the quality of an obtained polycarbonate can be guaranteed because the polymerization occurs in a solid state. In general, however, the crystallization and the solid state polymerization are performed without removing reaction side products having a degree of polymerization of less than 3 and non-reacted diarylcarbonate which co-exists with a relatively low-molecular weight prepolymer (weight average molecular weight of 2000 to 20000 g/mol). As a result, the difference between moles of an aromatic group and arylcarbonate group is increased, thereby prolonging the manufacturing time for a high-molecular weight polycarbonate.
Accordingly, there is a need to develop a polycarbonate manufacturing method that is stable, guarantees quality, and requires only a short time to produce a high-molecular weight polycarbonate.